Crystalline materials

ABSTRACT

A COMPOSITION OF MATTER TRANSPARENT TO INFRARED RAYS AND SHOCK RESISTANT WHICH IS A DIRECTIONALLY GROWN, POLYCRYSTALLINE SOLIDIFIED MELT OF A EUTECTIC COMPOSITION COMPRISING AT LEAST TWO COMPONENTS WHEREIN THE RATIO OF THE INDEX OF REFRACTION OF ONE TO THE OTHER IS WITHIN THE RANGE OF 1.2 TO 1.0, THE DIRECTIONALLY ORIENTED, POLYCRYSTALLINE MASS IS GROWN IN A CONTROLLED MANNER BY ADDING HEAT TO THE MELT AND REMOVING THE SAME FROM THE SOLIDS DURING THE SOLIDIFICATION STEP, THE AMOUNT OF HEAT BEING ADDED TO THE MELT AND CONDUCTED THROUGH THE SOLIDIFIED MELT BEING 10**2 TO 10**5 TIMES THE LATENT HEAT OF FUSION OF THE EUTECTIC MIXTURE.

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w w w.. .w ...w u Czmum woztwz CARL F. SWlNEHARTa/va MARKO SFILIGOJ INVENTORS BYw/W/M il i, 1973 c. F. SWHNEHART Erm. 3,766,080

CRYSTALLINE MATERIALS Filed Oct.

2 Sheets-Sheet 2 ,m AcTUAToR CONTROL I 27a f3a U30@ @35a azzaa Maa AME RELAY RUDDER Ll: ELEVATION CONTROL 246 ACTUATOR Mba E LEVATOR CAQL F. SWNEHARTAND MARKO SF/LIGOJ, INVENTORS United States Patent Office 3,766,080 Patented Oct. 16, 1973 Int. Cl. G02b 5 /20 U.S. Cl. 252-300 6 Claims ABSTRACT F THE DISCLOSURE A composition of matter transparent to infrared rays and shock resistant which is a directionally grown, polycrystalline solidified melt of a eutectic composition comprising at least two components wherein the ratio of the index of refraction of one to the other is within the range of 1.2 to 1.0, the directionally oriented, polycrystalline mass is grown in a controlled manner by adding heat to the melt and removing the same from the solids during the solidification step, the amount of heat being added to the melt and conducted through the solidified melt being l03 to l05 times the latent heat of fusion of the eutectic mixture.

RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 314,952, filed Oct. 9, 1963, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to new bodies having infrared radiation transparency, a property not possessed by other bodies of identical chemical composition. More particularly, this invention relates to a multicomponent, multiphase, polycrystalline mass of controlled grown crystals substantially free of voids, comprising at least two different solid phases in substantially continuous atomic contact with each other. The instant grown crystals which are multicomponent and multiphase are by definition also polycrystalline.

The polycrystalline bodies of the present invention comprise multiphases and multicomponents in the sense used by Willard Gibbs as extended by Smits (see Findlay, The Phase Rule, Longmans Green, New York 6th edition 1927, pp. 6, 7, and 28). In the multiphase, multicomponent polycrystalline bodies of the present invention, the phases differ in atomic structure or lattice spacing, and the components, of which there are more than one, conform to the definition for equilibrium (Findlay supra).

Constituents which may be used to prepare the bodies of the present invention are represented by the general formula:

MmXn

of the present inventlon are prepared by conventional crystal-growing techniques, used heretofore to grow single phase, essentially monocrystalline bodies from a melt. The necessary proportions of two or more constituents, capable of forming a multicomponent eutectic, are mixed intimately and grown by any one of the well known crystal growing procedure, such as the Stockbarger-Bridgman procedure (U.S. Pat. No. 2,149,076), the Kyropoulos-Czochralski procedure [Z. Phys. Chem. 92, 219 (1917)] or the Verneuil procedure (CR. 135, pp. 791-4, 1902).

The bodies as prepared according to the instant invention are characterized by an intimate matrix of large, visible crystals in the size range from about microns to about 5 mm. in cross section and arbitrary length, generally several times the cross section limited only by the length of the ingot grown. They -do not cleave, are resistant to thermal shock and impact, and approach maximum density for the overall composition.

A preferred class of multicomponent, multiphase, polycrystalline bodies may be prepared according to the present invention having good to excellent transmission of light of selected wave lengths by choosing materials having very similar refractive indexes. Depending on the composition substantial percentages of specular (collimated) transmission are observed at different wave length ranges of the spectrum. In order to obtain a significant amount of specular transmission, i.e., in excess, of about 20 percent transmittance, the components of the instant composition should be chosen so that the ratio of their refractive indexes falls within the range of from 1.0 to 1.2, and preferably within the range of from 1.0 to 1.05, for the larger over the smaller. For usable diffuse transmission such as is permitted in a scintillator or fluorescent light source, the range for the ratio of refractive indexes of the separate phases can be broader, including ratios as high as 1.5 or even higher.

It has been found that the uorides of the elements of Groups II, III, IV, V, and VI of the Periodic Table (Handbook of Chemistry of Physics, 40th edition, page 444, Chemical Rubber Publishing Co., Cleveland, Ohio) particularly are useful as constituents for preparing the light-transmitting bodies of the present invention, especially light having wave lengths in the infrared range. Almost all of these iluorides are substantially water-insoluble and, when comprising the components of a multiphase, optically integral crystal, transmit a large amount of light, particularly in the infrared range.

It is unexepcted that the bodies prepared according to the present invention are capable of transmitting collimated light in view of the fact that the identical components when fused and cast as an integral body uniformly are opaque. Bodies of the present invention which are pervious to infrared radiation may then be properly termed as being optically integral. Optically integral is used in the sense that there is substantially an optical coupling between the parts of the whole body providing an infinite number of light paths having much the same efficiency throughout the integral body, either where light is transmited from one surface of the body to another, as in a window, or from a point within the body to a surface of the body as in a scintillator.

Certain of these optical bodies which transmit wave energy extremely well, especially in the infrared, may be used in many areas where single crystals cannot be used because of their intrinsic nature; the crystals may bend in a certain direction, cleave under impact, fracture with thermal shock, and the like. A preferred polycrystalline body prepared according to the present invention consists of barium uoride and calcium iluoride, each of which constitutes a separate phase from the other within the body. These materials conduct light energy extremely Well in the range of wave lengths from about 1.5 microns to about 12 microns at a thickness of 5 mm.

The light-transmitting bodies of the present invention have many advantages over the pressed optics which are prepared by hot pressing small, randomly oriented, granular crystals into an integral matrix to form an optical window. Pressed optics are used in various areas for transmitting light because they are not susceptible to cleavage. Because of this random orientation, they are more resistant to thermal shock and impact and therefore may be used under conditions which would cause crystals to break or cleave. The pressed optics in many ways are similar to bodies of the present invention by way of their physical properties. These pressed optics, however, are limited as to their degree of purity and practically as to size. Because of high surface area in the granular mass of small crystals which is used to obtain the pressed optic, it is extremely difcult to acquire a finished product of maximum purity. The mechanical confinement of the die substantially suppresses the vaporization of almost all of the light-absorbing impurities and renders any scavenger additives useless. Moreover, because the optical prisms must be hot-pressed in a die, they are limited technologically by the size of the die and press and the ability of the die to control the density throughout the entire mass of small crystals. Various glass-forming compositions have been investigated for developing infrared transmitting bodies, but the best compositions only transmit infrared having wave lengths up to 6 microns and invariably absorb infrared radiation in the 3 micron region.

The preferred barium uoride-calcium fluoride polycrystalline bodies of the present invention advantageously may be cut or ground into various optical shapes and used with various light detecting devices and systems which are frequently subjected to shock or other rough treatment. Their size is not influenced by the expense of making a die for each particular application. Moreover, the bodies of the present invention may be made almost any size without internal strains, internal cracks, open boundaries between components and the like, whereby the bodies may be cut, ground or otherwise formed into any shape or configuration. They are particularly useful as a nose window for wave-energy guided vehicles such as the inrared homing missiles and the like. Because of their unique physical properties, the multiphase polycrystalline bodies of the present invention particularly may be adapted for such rugged use, cooperating in a unique manner (excellent optical cooperation) with infrared detecting and guiding systems employed with such vehicles and the like.

SUMMARY OF THE INVENTION It has been discovered that a melt of at least two components capable of forming a eutectic composition, wherein the ratio of the index of refraction of one to the other is within the range from about 1.2 to 1.0, may be directionally solidified to form a polycrystalline mass of threedimensional elongated crystals oriented in a direction normal to the solidification front, under controlled growth conditions so that the polycrystalline mass is transparent to infrared radiation. Though the instant polycrystalline mass is grown from an essentially eutectic composition, none f the crystallites has the eutectic composition, but each elongated crystal is a crystallite of one of the phases. The controlled growth of the new composition, which has no voids at the crystallite boundaries, permits light transmission, which is not possible with casting.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph plotting the percent light transmission at various wave lengths of a barium fluoride-calcium fluoride mixture capable of forming a eutectic and, upon cooling, an optical body of the present invention.

FIGS. 2 and 3 are graphs similar to the graph of FIG. 1, comparing the light transmission of additional compositions, the rst one being prepared according to the present invention at an excessively rapid growth rate, and the second prepared by casting techniques.

FIG. 4 illustrates an air-to-air infrared homing missile with a hemispherical window comprising its nose.

FIG. 5 illustrates a hemispherical window such as may be prepared from the bodies of the present invention in combination with a guidance system advantageously used with the missile of FIG. 4.

PREFERRED EMBODIMENT OF THE INVENTION As indicated hereinbefore, it is important that at least two of the constituents (compounds or elements) comprising the components used to prepare the bodies of the present invention be capable of forming a eutectic. The proportions of these constituents are not restricted to proportions close to or at the eutectic composition. However, at least 8 to 10 percent of the constituent constituting the minor phase should be used, and preferably the proportions of the constituents constituting the minor phase or the major phase should be within 15 percent by weight of the eutectic. In order to obtain an optically integral body, it is especially preferred that the proportions of these constituents be held to within about 5 percent of the eutectic and still permit one phase to start to grow before the other. It is advantageous to have a slight excess of one constituent over the proportion of that component in the eutectic.

That the proportions are not restricted to those of the eutectic is apparent from the fact that the phases within the bodies having a substantially uniform composition throughout are obtained even though there is a disproportionate evaporation of the components from the melt during the growth. There is a slight relative change in volume of the two phases, but the amount of evaporation is so small that any difference in relative volumes of the phases may be considered insignificant or inconsequential.

The small variance in composition found in bodies made according to the present invention is in apparent conflict with the Phase Rule until it is realized that the bodies are not formed under true equilibrium conditions. The Phase Rule indirectly requires that eutectics have a specific composition because at equilibrium the melt can exist only as a single composition when in contact with dilerent solid phases equal to the number of components. In the instant situation, however, the melt is superheated and there is usually a disproportionate evaporation of constituents during growth.

The essential feature of the present invention is the growing of the instant bodies under controlled conditions. The rate at which the multiphase polycrystalline bodies of the present invention are grown depends on the temperatures of the two zones and the flow of heat through the growth interface. The rate of growth, Crucible size, and the inside diameter of the crucible must be considered in determining the proper conditions necessary for obtaining a body having the desired physical characteristics. These factors are, of course, well known to those skilled in the art, and need no further discussion.

Preferably, the melt should be mixed before and continually during the progressive solidication of the melt. When the Stockbarger procedure (supra) is to be used,

a positive means of stirring may be necessary because there is little convection mixing, heat being applied from above. In the Kyropoulos procedure (supra), mixing is inherent either from rotation of the solid as it is grown on the surface or from heat addition to the lower part of the melt.

Discontinuities in the materials comprising the charge move toward the heat source at a rate dependent upon vapor pressure of the material and the thermal gradient maintained in the solid. Where optical bodies are to be produced, the growth rate is especially important; these discontinuities, regardless of form, affect the light transmission characteristics. Thus, for best results, the growth rate should be several orders of magnitude slower than the flow of heat would permit. In other words, the amount of heat being conducted through the solidifying mass is several orders of magnitude greater than (104-105) the latent heat of fusion. Y

I n melting and growing the material in a good vacuum, pressures less than 2 microns Hg absolute are preferred but pressures up to l0 mm. Hg absolute are tolerable. Atmospheres of the noble gases including argon, helium, and the like, and hydrogen, can be used with the Kyropoulos procedure. However, trace amounts off oxygen or nitrogen usually contaminate the bodies being prepared for optical purposes, especially those prepared from a uoride-containing melt; the resulting bodies usually contain color absorption bands and light scattering imperfections.

Specilic elements represented by M of Formula I particularly include aluminum, barium, cadmium, calcium, cesium, chromium, cobalt, copper, germanium, indium, iron, lead, lithium, magnesium, molybdenum, potassium, rubidium, silicon, silver, sodium, strontium, titanium, zinc, zirconium, and the like. Anions advantageously useful in combination with these metals, represented by X in Formula I, particularly include hydroxyl, carbonate, sulfate, nitrate, sulfide, selenide, telluride, fluoride, chloride, bromide, iodide, oxide, cyanide, arsenide, thiocyanate, chlorate, iodate, phosphate, tungstate, and the like. The glassforming anions, such as borate, silicate, titanate, zirconate, and the like, in general, when combined as a salt, do not form a eutectic, and therefore do not fall within that scope of the present invention.

Polycomponent systems are also considered to be Within the scope of the present invention and particularly include ternary and quaternary systems comprising compounds represented by Formula I, and include those compounds made from the above-listed metals and anions. As previously mentioned, the systems must include a eutectic, and the individual phases must transmit light to have a refractive index. Other compounds which are soluble in one or more of the phases comprising the ternary, quaternary or other systems may be used also.

Specific examples of binary eutectic systems useful in preparing the multicomponent, multiphase polycrystalline bodies of the present invention are set forth in the following Table I:

TABLE I Mol percent Melting Component II of Component point ofmix- I eutectic II ture, "C

SbQSg 25 SbzOa 485 LiF 80 LOH 430 LiCl 28 LiF 498 LiBr N13 LiF 453 Ll2SO4 43 LiF 535 LizSOl 63.5 LiCl 478 LiNOa 11.8 LiCl 252 LiNO; 13 LizSOi 252 Li2CO3 49 LiF 608 LiQCOg 60 LiCl 506 LigCOg 60 Li2SO4 540 LizCOa 98 LINO; 250 NAF 61 LiF 652 NaCl 33.5 NaF 676 NaBr 23 NaF 642 NaI 18 NaF 603 aI 37 NaCl 570 NaI(T1) -37 NaCl 570 KF 50 LiF 492 KF 40 NaF 710 KCI 26.5 NaF 648 KCl 45 KF 606 KBr 21 NaF 650 KBr 40 KF 576 KI 12 NaF 631 TABLE l-Continued Mol percent Melting Component II of Component point ofmix- I eutectic II ture, C.

KI 43.5 NaCl 514 KI 34 KF 544 KI 47.5 KCI 598 RbF 50 LiF 450 RbF 33 NaF 644 RbCl 46 NaCl 541 RbCl 57 RbF 545 CsCl 58.5 LiCl 332 CsCl 35 NaCl 493 MgO 60 BeO 1,855 MgBr2 59 NaBr 431 MgS 33 MgO 1,600 cao es Beo 1,470 CaO 45 MgO 2,280 CaFz 86 LiF 765 CaFz 68 NaF 810 CaF2 37 MgO 1,340 CaF2 57 MgFz 945 CaClz 47 NaCl 500 O 50 MgO 1,935

SrClg 50 NaCl 565 BaF2 65 NaF 825 BaFz 72 KF 750 BaFz 92 KCI 740 BaFz 79 RbF 665 BnClz 61 NaCl 640 BaClz 21 CaFz 791 BaBrz 60 NaBr 600 ZrOz 49 MgO 2,070 Zr02 50 A1203 1,885 ThOz 65 BeO 2,200 T1102 60 CaO 2,300 F6012 44.5 CaClz 592 FeClz 50 SrCla 541 FeCls 46 NaCl 158 Fes 23 oas 1,120 FeS 6.5 MnS 1,164 C 42 Cao 1,425 NizS 29.6 NagS 615 NlsSz 41.4 Naas 647 C1120 47 CaO 1,140 CuCl 27 NaCl 314 CuI 57 CuCl 284 Cu2S 93.5 Ni2S 575 CuzS 67 Ni3S2 728 CutS 88.7 CuCl 391 AgC 30 KCl 318 Ag() 25.1 KBr 320 AgCl 40 RbCl 251 AgCl 9 MgCl2 455 AgCl 30 CsCl 258 AgCl 54 CuCl 260 AgCl 27 CuzS 318 AgBr 32 KBr 285 AgI 40 NaI 394 AgI 43.4 C1101 216 Agr 53 Agoi 264 AgzS 59.6 AgCl 376 AgaS 91 AgBr 392 ZnF2 NaF 682 ZnS 93.6 FeS 1,180 ZnS 90.4 AggS 807 AgI 20.8 KI 254 Agr 28.5 Kr 244 The examples of several ternary systems which may be grown into the multiphase polycrystalline bodies of the present invention are set forth in the following Table II:

The starting materials which are used to prepare the multiphase polycrystalline bodies of the present invention should be fairly pure and substantially free from water. Various standard procedures known to those skilled in the art for purifying the starting materials of the present invention may be used. After the reactants are upgraded sufficiently, they are then blended or mixed in the desired proportions and grown according to any of the methods previously mentioned. Inasmuch as the majority of the starting materials used to prepare the bodies of the present invention are salts, a simple preparation from the appropriate base, such as a hydroxide or carbonate, and the appropriate acid or acid salt may be used. Reaction rnay be effected in an aqueous or anhydrous medium according to methods well known to those skilled in the art. Of course, scrap crystal ingots or the naturally occurring mineral may be used, Moreover, when they are grown for optical purposes, it is preferred that a scavenger be used. Particular scavengers useful with the alkaline earth metal fluorides are lead fluoride and the vapors of manganese trifluoride and cobalt trifluoride, Teachings of their use will be found in U.S. Pat. No. 2,498,186 to Stockbarger and U.S. Pat. No. 3,282,614 to Sfiligoj and Swinehart, respectively.

In order that those skilled in the art may better understand how the present invention can be carried into effect, the following examples are given by way of illustration, and not by way of limitation. All parts and percentages are by weight unless otherwise specified.

EXAMPLE I Optical scrap barium fluoride (367 parts) 60.8 mole percent and optical scrap calcium fluoride (100 parts) 37.2 mole percent is mixed with lead fluoride (10` parts) 1.2 mole percent and charged into a graphite Crucible. The crucible is then covered with a lid provided with /e" vent hole. Manganese trifluoride (3 parts) 0.8 mole percent is placed in a small cup aflixed to the underside of the lid and provided with a small aperture to provide gaseous communication between the cup and the Crucible. The furnace (Stockbarger type) is then evacuated to a pressure of less than 0.1 mm. Hg absolute, and heat is applied over an l8-hour period until all gassing ceases and the charge fuses into a melt and until the temperature reaches about 1300 C. After setting the temperature controls down to growth as indicated by the location of the solid liquid interface at the temperature gradient zone, the crucible is lowered slowly into a cooler zone at 4 mm. per hour and continued for 24 hours. The elevator is stopped and the temperature slowly brought to room temperature over a period of 24 hours. After cooling, the ingot is removed from the crucible and laterally sliced perpendicular to the longitudinal axis into a series of discs 5 mm. thick. After polishing, each of the discs is tested for its optical characteristics using a Perkin Elmer Model 21 or infracord 137 spectrophotometer. 95 percent of the discs have infrared transmission properties substantially as represented by FIG. l of the drawing.

EXAMPLE II Example I is repeated, using a charge comprising (500 parts) 68.0 mole percent optical scrap barium fluoride, (100 parts) 30.6 mole percent optical scrap calcium fluoride, (12 parts) 1.2 mole percent lead fluoride, and (l part) 0.2 mole percent cobalt trifluoride in the cup.

EXAMPLE III Example I is repeated, using a charge comprising (190 parts) 60.2 mole percent of precipitated barium fluoride, (52 parts) 37.0 mole percent of selected fluorspar, (10 parts) 2.3 mole percent of lead fluoride, and (1 part) 0.5 mole percent of manganese trifluoride in the cup.

EXAMPLE IV Example I is repeated, using a charge comprising (367 parts) 61.3 mole percent optical scrap barium fluoride, (l0 parts) 37.5 mole percent optical scrap calcium fluoride, and parts) 1.2 mole percent of high quality lead fluoride. There was no manganese trifluoride placed iu the cup at the underside of the cover.

All of the compositions prepared according to the foregoing Examples (I-IV) have physical properties falling within the following specifications,

PHYSICAL PROPERTIES Specular (collimated) transmission:

Usable 1.5 to 11.4 microns, over 10% for 5 mm. thickness.

Excellent 3 to 9 microns, over 60%; .25 to 1.5 microns, over 20% visible range for 5 mm. thickness.

Optical absorption bands None.

Appearance White, translucent, marble-like.

Density 4.50 to 4.44 gm./cc.

Hardness (mohs scale) 4.5 (higher than aCF2 or BaFz).

Cleavage None.

Fracture Irregular to concoidal.

Strength and rigidity Comparable with porcelain or marble.

Thermal shock resistance 5 x 15 x 20 mm. slabs show no change upon repeated quenching (10 cycles) from 300 C. into ice water or onto Dry Ice.

Thermal expansion 25 108/ C.

Solubility Less than 2 parts per thousand in water at 30 C., none in organic solvents.

EXAMPLE V Example l is repeated, except that the Crucible is stopped after two-thirds of the melt has been solidified into a multiphase polycrystalline mass of the present invention. The furnace is then allowed to cool to room temperature, permitting heat to be withdrawn from the melt through the walls of the Crucible, and consequently allowing the upper portion of the melt to solidify as a cast mass. The resulting ingot is then cut laterally into thin portions perpendicular to the longitudinal axis and tested for infrared transmission. The curve of FIG. 2 was obtained from the portion of the ingot immediately below the cast mass where growth was rapid, but before the application of heat to the melt was withdrawn. The curve of FIG. 3 was obtained from the upper cast portion of the ingot.

By comparing the graphs of FIGS. 1, 2, and 3, the optical integrity of a preferred multicomponent, multiphase polycrystalline body (barium fluoride-calcium fluoride) of the present invention becomes significant. Crystals of either barium fluoride or calcium fluoride are well known optical materials; they transmit light of various wave lengths extremely well. When these two materials are fused together, cast (solidified as set forth in Example V), cut to a 5 millimeter thick body and polished to form a window, the window substantially is opaque, transmitting little if any collimated light, especially in the infrared below 12 microns. As previously indicated, the graph of FIG. 3 represents the infrared transmission of such a body; the curve shows substantially zero infrared transmission for the cast body.

As set forth in Example V, part of the same melt was cast by moving the melt into a cooling zone while the remainder was held in the crucible and grown to form a crystal ingot. This grown material and the sound part of the casting were cut and polished to form 5 mm. test plates. The transmission of the grown material approximated that of FIG. 2 or FIG. 1, while the cast portion was zero, as in FIG. 3.

The graph of FIG. l is an infrared curve showing the precent transmission of an optical body grown according to Example I. This curve shows excellent transmission of infrared from about 1.5 to about 11.5 microns for an optical window mm. thick. Moreover, these bodies made therefrom, when polished, transmit between 80 percent to about 90 percent of the infrared in the range of from about 2 to over 8 microns. Bodies grown under less favorable conditions as to rate or thermal gradient may approximate FIG. 2.

EXAMPLE VI When Example I is repeated using (40 parts) 8.5 mole percent strontium liuoride added to the barium fluoride 55.7 mole percent, calcium fluoride 34.0 mole percent, lead iiuoride 1.1 mole percent and manganese trifluoride 0.7 mole percent, a body having good optical characteristics is obtained.

EXAMPLE VII When Example I is repeated, using (533 parts) 17.5 mole percent optical scrap calcium fluoride and (9467 parts) 82.5 mole percent purified calcium iodide prepared as below, a polycrystalline mass is obtained having a matrix of large scintillation crystals optically coupled to large crystals of calcium fluoride.

Preparation of calcium iodide In a suitable vessel of glass, stainless steel, or platinum, hydriodic acid (47 percent solution) 790 parts is added to calcium carbonate (215 parts) stoichiometrically in excess of the calcium, boiled to remove CO2, and made alkaline wtih calcium oxide. The resulting reaction mass in the form of a slurry is filtered for insolubles and made acid with additional 47 percent aqueous hydrogen iodide to a pH below I. The filtrate is boiled to concentrate the solution through a continually rising boiling range to 146 C. The resulting highly concentrated liquor was cooled to 5 C. to form calcium iodide hexahydrate in crystalline form. The mother liquor is poured olf and the crystals are spread loosely in an inert tray of quartz or a resinous material, such as polytetrauoroethylene, to a depth of approximately 1 inch. The lled tray is then placed in a vacuum drying oven equipped with a vapor trap cooled with Dry Ice. The oven is then evacuated and pumping continued over a period of 24 to 48 hours, with no heat applied, until a pressure of 0.1 mm. Hg absolute or less is attained. The calcium iodide is then heated slowly to about 75 to 80 C. over an interval of 40 ot 60 hours, and held at this temperature for approximately 24 hours, or until a pressure of about 0.1 to 0.03 mm. Hg absolute is attained as measured by a Prani-type vacuum gauge.

EXAMPLE VIII When Example I is repeated, using (18.5 parts) 36.9 mole percent optical scrap sodium chloride and (80.6 parts) 67.8 mole percent sodium iodide, (0.9 part) 0.3 mole percent thallium iodide, a body comprising a matrix of scintillation crystals similar to that obtained according to Example VII is obtained.

EXAMPLE IX When Example I is repeated, using (100 parts) 37.1 mole percent calcium iiuoride and (367 parts) 60.6 mole percent barium fluoride, parts) 1.19 mole percent lead uoride, (3 parts) .078 mole percent manganese triuoride and (3 parts) .032 mole percent europium triiluoride trihydrate, a body comprising a matrix of scintillation crystals similar to that obtained according to Example VII is obtained.

Windows and other optical bodies may be made from the multicomponent, multiphase polycrystalline bodies prepared according to the present invention using conventional grinding techniques. Examples of such techniques include (l) abrasive wheels having diamonds or other abrasives impregnated or bonded in the surface thereof, (2) mechanically driven copper or other soft metal cutter blades running in slurries of an abrasive such as silicon carbide or emery in a liquid, and (3) ultrasonic driven steel or Monel cutters in similar slurries of abrasives.

The windows or optical bodies of the present invention may be aixed to a supporting structure using conventional methods employed in mounting crystals or glass windows. Because of their relative ruggedness, these bodies may be clamped around a peripheral flange integral with the window, or they may be bonded with an adhesive or cementitious material, such as an epoxy resin. Fused silver chloride has been exceptionally adaptable as a bonding agent for crystals (see J. Kervitsky, Report of N.R.L. Progress, February 1960).

The optical bodies of the present invention particularly are used advantageously as windows for devices which are responsive to infrared radiation. Generally, these devices are based on a simple circuit including an element in series with an impressed voltage wherein the element changes its resistance when exposed to infrared radiation. The changes in voltage in the circuit due to the change in the resistance of the infrared element are correlated easily with the amount of infrared radiation. Commonly, the infrared sensitive element comprises a semiconductor material having a high negative temperature coeflicient of resistivity. Such elements usually are called bolometers The following formulae set forth the principle of the circuitry involved:

wherein V is the total voltage across the entire circuit, V1 is the voltage measured at the output and V2 is the voltage across the bolometer; R1 is the resistance across the circuit and R2 is the resistance across the bolometer. It is easily seen that the voltage V1 is dependent on the value of the resistance R2. As the resistance R2 decreases because of the effect of infrared radiation, the voltage V1 correspondingly increases and as the resistance R2 increases, the voltage V1 correspondingly decreases, the voltage V across the entire input circuit remaining constant. The sensing elements or bolometers are of minimum mass in order to have an adequate response. However, these elements are susceptive to corrosion and are very responsive to air currents; therefore, to be used in most instances, they must be protected by transparent windows.

Because of their resistance to shock and because they are not handicapped by cleavage problems, the infrared transmitting bodies of the present invention are particularly useful as windows in missiles guided by systems responsive to infrared radiation coming from a target. Such missiles or vehicles include steering systems such as th'ose set forth in U.S. Pats. No. 2,826,380 and 2,897,369, which act to guide the missile to its intended destination.

Referring to FIG. 4, there is shown an infrared homing missile 10 which may advantageously use a window made from a multiphase polycrystalline body prepared according to the present invention. The missile 10 comprises an elongated cylindrical body 12 having a rear section 14 containing the steering activating system and a front section 16 containing the infrared detector and signal 4former and the warhead or explosive. The steering activating system in the rear section 14 is controlled by the signals generated by the infrared detector.

In the nose of the missile l() is window 18 of spherical configuration for transmitting infrared radiation. At the other end of the missile is a rudder and elevator combination 22. Axially spaced therefrom toward the center of the missile is a stabilizer system 20.

Referring to FIG. in conjunction with FIG. 4, there is shown a system adapted to effect steering of the missile optically coupled to and in optical cooperation with the window 18. The system comprises two similar portions, one for controlling a rudder 24 to steer the body horizontally and the other for controlling an elevator 24a to steer the body 12 vertically. In the drawing, each of the elements of the elevator 24a controlling portions is designated by the same reference number combined with the letter a as the corresponding element of the rudder control portion.

The latter comprises an actuator 26 for defiecting the rudder in one or the opposite direction in accordance with information as to the bearing of a source of radiant energy, e.g., a target, derived by way of a scanner 27, which is actuated by a bearing control 28 to scan a field of prescribed angular extent, repeatedly and in the horizontal direction. The scanner 27 is arranged to be tilted normal to the scanning direction by an elevation control 40. The scanner 27 comprising a substantially planar mirror is positioned in optical relation to a parabolic reflector for channeling the infrared waves R to impinge on a detecting element 30, such as a bolometer or the like.

The detecting element 30 is in series with a battery 31 providing input for the amplifier 32. A suitable resistor 34 and a capacitor 35 are included with the battery 31 and detecting element 30 to provide an input circuit capable of feeding the desired signal to the amplifier 32.

A relay 33 controlled by the amplified signals coming from the amplifier 32 controls the activator and the bearing control.

In the elevator control portion of the steering system, the scanner 27a is actuated by the elevation control 40a to scan substantially the same field as the scanner 27, but in the vertical direction, and is arranged to be tilted normal to the direction of its scan by the bearing control 23a.

The bearing control 28a is controlled by the relay 33 in accordance with signals received from the amplifier 32. Similarly, the elevation control 40 is controlled by the relay 33a in accordance with signals received from the amplifier 32a. The elements of the system, it will be understood, are mounted on the body to be steered so that the scanners view the field ahead of the body.

Briefiy, the general operation of the system and the functions of the elements thereof are as follows: The scanners 27 and 27a scan a field of relatively large extent and are associated with the respective detector in such manner that when the energy received by the scanner is uniform throughout the scanning cycle, no signals are produced by the detector. When, however, an energy discontinuity, due to for example the presence of a target in the field viewed, is encountered by the scanners, the response of the detecting element 30 causes changes in the signal coming from the amplifier determined by the bearing of the target relative to the body and the response of the detecting element 30a causes changes in the signal coming from the amplifier 32a determined by the elevation of the target. Each signal is applied to the respective actuator to cause deliection of the rudder or elevator accordingly.

The signal coming from the amplifier 32 is applied to the bearing control 28 in such manner that the field covered by the scanner 27 is restricted to one of small angular extent to either side in the horizontal dimension of the line from the target to the body. Similarly, the signal coming from the amplifier 32a is applied to the elevation control 40a in such manner that the field covered by the scanner 27a is restricted to one of small angular extent to either side, in the vertical dimension, of the line noted.

Also the signal coming from each of the amplifiers 27 and 27a is applied to a tilt control for the other scanner in such manner that the horizontal scanner 27 is aligned vertically with the target by operation of the elevation control 40 and the vertical scanner is aligned horizontally with the target by operation of the bearing control 28a. Thus, when a target appears in the field, the scanners are actuated to such positions that the target is centered in the field viewed by the scanners and the scan in both horizontal and vertical dimensions is restricted, whereby accurate information at relatively high frequency as to the bearing and elevation of the target is obtained.

The optical bodies of the present invention may be used advantageously with other devices depending upon infrared radiation to actuate a primary or auxiliary system of the device. They may be used in devices which convert infrared light to visible light, such as snooperscope. They may be used with infrared cameras, either as protective windows or lenses for the camera. They are also used advantageously as lenses for telescopes, eliminating the need for reflector mirrors that are difficult to streamline or adapt to certain geometrical alignments of lens, tube, and sensing element. Mosaics or matrixes of scintillator materials and/or thermolumiuescent materials with non-active or less active components are readily made according to the present invention. In this form, the bodies of the present invention are advantageously applied as fluoroscopes, light sources and the like, wherein the exciting radiation can come from without or a doping or radioactive constitutent of the body itself.

While specific examples of the invention have been set forth hereinabove, it is not intended that the invention be limited solely thereto, but to include all of the variations and modifications falling within the scope of the appended claims.

We claim:

1. A non-cleaving optically integral multi-component multi-phase polycrystalline body of controlledly grown crystals, directionally oriented in a direction normal to the solidication front of a eutectic melt from which they are grown, said body having at least two individual and distinct phases, each phase being present as a threedimensional elongated crystal, wherein the ratio of the index of refraction of one to the other is within the range from about 1.2 to 1.0 imparting to said body transparency to infrared radiation, said individual phases being represented by the general formula MmXn, wherein M represents an element chosen from Group I, II, III, IV, V or VII of the Periodic Table and X represents an anion; wherein m is an integer ranging from l to 6 and n is an integer ranging from 0 to 6.

2. The polycrystalline body of claim 1 wherein said eutectic melt composition consists essentially of each component present in the proportion required for forming said eutectic composition, and in addition a slight excess of one component over the other with respect to said amount, said component present in excess crystallizing as an individual phase in an end portion of the solidification zone prior to the solidification of directionallyoriented, individual, three-dimensional elongated crystals of said each component to form said polycrystalline body characterized by being pervious to specular infrared radiation so as to exhibit a transmittance in excess of 20 percent.

3. The composition of claim 1 wherein the components forming a eutectic melt are selected from barium fiuoridecalcium fluoride; lithium fiuoride-sodium iodide; antimony sulfide-silver sulfide; lead oxide-lead selenide; indium phosphide-indium antimonide; lead telluride-lead sulfide.

4. A new composition of matter, transparent to infrared rays and resistant to thermal shock, the same being a solidied melt of two components present in proportion approximately eutectic, one of said components being BaF2 and the other being CaF2.

5. The composition of claim 1 wherein controlled growth of said directionally-oriented, individual, threedimensional elongated crystals in said polycrystalline body is effected by maintaining a heat ux through said solidication front in an amount in the range of from 103 to l()5 times the latent heat of fusion of said eutectic mixture.

6. rIhe composition of claim 1 wherein said polycrystalhne body consists essentially of individual phases of ionic salts, said body being capable of transmitting at least 20 percent of incident light having a Wave length in the range from about 1.5 to about 15 microns in a thickness of 5 millimeters.

References Cited UNITED STATES PATENTS 2,498,186 2/1950 Stockbarger et al. 23-88 2,642,335 `6/1953 May et a1. 23-303 X 2,739,045 3/ 1956 Pfann 23-29'3 X 2,966,592 12/ 1960 Vogl et al. t 350-1 X 3,111,088 11/1963 Fisk 244-14 X 3,002,811 10/1961 Johnson 23-89 10 NORMAN G. ToRCHIN, Primary Examiner E. C. KIMLIN, Assistant Examiner U.S. C1. 

